Abstract Substance use disorder (SUD) is the second most common psychiatric diagnosis in the U.S. population, with 14.6% developing it in their lifetime. Transition-age youth (TAY) from ages 18-25 are especially vulnerable to substance use and SUD. This time period, termed the ?age of instability, is now understood to represent a distinct stage separate from adolescence and later adulthood. It includes tasks such as leaving home to enter college, the workforce, or the military; clarifying vocational goals; attaining legal drinking age; and, for some, aging out of foster care or state custody, which end at age 18. This period is also prime for experimentation with substance use and associated risks such as driving under the influence, accidents, fighting and violence, HIV, and vulnerability to sexual assault. In this project, we focus on a major strategy, grounding, to help TAY de-escalate (reduce) intense negative emotions and impulses. Grounding has long been used in psychiatric hospitals to provide a safe, quick, and powerful way to reduce any intense negative feeling such as impulses to hurt self or others, substance craving, anger, anxiety, etc. Described in detail in the book Seeking Safety (SS), grounding using three pathways to focus the mind (mental), body (physical), and heart (soothing). Grounding provides a way to regulate emotion, which is a major challenge for TAY, especially given their hormone changes and emotional intensity. Indeed substance use is often described as a short-term way to regulate emotion. Grounding is a key module of SS, a widely-implemented, evidence-based SUD treatment model. In this proposal we focus exclusively on grounding as it is an essential skill and widely identified as easy to learn for even the most complex and vulnerable clients. In phase 1 we developed a beta version of a grounding app. The app promotes the use of grounding in any environment in which emotions and impulses need to be regulated. It capitalizes on TAYs? widespread use of mobile devices; and because grounding is a stand-alone skill and sensory-oriented, it lends itself wonderfully to an app approach. We used an iterative product development methodology based on end-user feedback and studied the impact of the app in a pilot randomized controlled trial (RCT) with 24 TAY. Half received the app and the other half received grounding handouts. Results showed that the app was consistently more positively rated than the handouts and was associated with significantly more use of grounding and a higher score on a grounding knowledge test. All phase 1 benchmarks were met. In phase 2 we propose to scale up both the app and the evaluation of it. We have two specific aims: (1) To enhance the mobile grounding app by using exciting, innovative technology approaches as well as expanding the grounding content in amount and type. (2) To conduct an RCT with 68 TAY to evaluate the grounding app versus a control app. The control app would control for both the use of a mobile device and the grounding content. However it would lack the gamification and technology innovations of the experimental app so that we can determine the impact of these. Our primary outcome is based on power analysis from our phase 1 pilot; we also include other outcomes and measures. The RCT uses rigorous scientific methods in its design, measures, and statistics. Our team represents an exceptional mix of expertise in SUD, TAY, app and gaming technology, grounding, SBIRs, and public health oriented products based on evidence-based care.